The gateless gate is regarded as the greatest book of zen ever complied. It is a collection of 48 Chan (Zen) koans compiled in the early 13th century by the Chinese Zen master Wumen Hui-k’ai (無門慧開; 1183–1260).

This book was so profound even during the Sengoku Jidai (戦国時代); Samurai (侍)and Shinobi (忍び) families were reading these words as an aid towards enlightenment. In the Shoninki written by Natori Masatake of the Kishu-ryu school of Ninjutsu he writes:

“One of the most important things for you to do is always know everything you can of every place or province that is possible to know. When you are in that said province, be sure to read the feeling of the local people and do what is necessary according to the situation. If your mind is in total accordance with the “way” and understands reasoning and logic perfectly, than you can pass through the mumon no ikkan (The Gateless Gate; 無門關) “checkpoint”. This is a mental place that is barrier without a gateway that exist inside your mind. You will be able to pass through this mental stage because you will have a clear mind and reasonable thinking due to your training.” — Natori Masatake; Kishu-ryu; Shoninki